The invention relates to a method and an apparatus for, the laser machining of workpieces, a laser beam being two-dimensionally positionable with the aid of a laser machining head with respect to a surface of a workpiece and it being intended for the machining of the workpiece to take place with predeterminable contours. The corresponding contours should as far as possible be in the form of closed lines, it being possible for a wide variety of geometrical shapes to be concerned, such as circles, triangles, squares, rectangles and others.
For a laser machining operation, usually one positioning of the laser machining head, with the aid of which a laser beam can be deflected and shaped, takes place by a corresponding relative movement with respect to the workpiece. The positioning.may in this case take place by two-dimensional movement of the laser machining head or of the workpiece alone or by simultaneous corresponding movement of these two elements.
If the laser beam is consequently to be guided or deflected over the surface of a workpiece in a way corresponding to the respective predetermined contour, in order for example to carry out a welded connection or some other targeted local heat treatment, or to produce a correspondingly contoured opening in the workpiece by a cutting operation, problems arise, in particular in the case of small-format and filigree structures, and the required positioning accuracy cannot in every case be ensured by the conventional drive mechanisms for the laser machining head or the workpiece, in particular at high machining speeds.
Furthermore, distortion which has occurred as a consequence of previous machining steps on the workpiece may also lead to corresponding dimensional and positional errors, so that the required positioning accuracy cannot be achieved in every case.
If, for example, hollow profiles, in particular thin-walled hollow profiles, are to be butt-welded into openings of a metal sheet by means of a laser beam, as far as possible without filler material, extremely high positional accuracy is required in the alignment and movement of the laser beam, so that the latter must be guided very accurately along the contour which is predetermined as the gap between [lacuna] corresponding to the outer contour and dimensions of the hollow profile and the opening formed in a corresponding way in the metal sheet, in order to be able to produce an adequately solid and sealed connection between the metal sheet and the respective hollow profile. The distortion mentioned may in this case already lead to corresponding positioning and alignment errors, so that production errors may occur with a set of hollow profiles to be welded if no correction of possibly occurring dimensional and positional errors is carried out.
Known positioning aids are, for example, separate markings which can be sensed by suitable sensors which are arranged externally. With the aid of such elements, a corresponding relative movement is generally initiated in order to correct the positioning error which has occurred, which leads to increased expenditure of time since this usually has to be carried out in several stages. With these means, however, it is only possible with difficulty, if at all, to compensate for positioning errors as a consequence of distortion.
If known image processing systems are used for monitoring the positional accuracy, the cost for a corresponding installation increases and an image sensing system required for this purpose must be arranged in such a way that it can monitor the respective machining area under all conditions, so that it is essential to evaluate a perspective image with correspondingly increased computing power.
It is therefore the object of the invention to propose possible ways in which high positional accuracy can be ensured in the laser machining of workpieces in predetermined contours at low cost and with little effort.
This object is achieved according to the invention by the features of claim 1 for a corresponding method and the features of claim 9 for a corresponding apparatus. Advantageous embodiments and developments of the invention are obtained with the features specified in the subordinate claims.
The solution according to the invention falls back on a laser machining head known per se, which can be designed for beam guidance and shaping, at least one scanner mirror allowing a deflection of the laser beam in two dimensions being used in each case. This may, for example, be a cardanically suspended scanner mirror, allowing a deflection of the laser beam in two axes aligned orthogonally with respect to one another over the surface of a workpiece.
For cost reasons, and possibly to increase the deflecting speed of a laser beam, it is, however, also quite possible to use two separate scanner mirrors which are arranged one after the other in the path of rays of the laser beam.
In this case, each of the scanner mirrors has a pivoting drive and an angle encoder, which are connected to an electronic evaluation and control unit, so that a defined pivoting of the scanner mirror or mirrors is possible by an electronic control, with simultaneous feedback, i.e. transmission of the respective angular positions of the scanner mirror(s).
Optionally, a focusing element may be arranged in the laser machining head, but also outside it, in the path of rays of the laser beam, it being possible, likewise optionally, for this to be a simple lens or a zoom lens.
Added to this, an additional light source, for example a laser diode, is used, the light beam of which can likewise be directed onto the scanner mirror or mirrors in a focused form.
With the aid of this light beam, an actual-set value comparison is carried out before the actual laser machining with respect to the positioning of the workpiece to be machined with predeterminable machining contours with respect to the laser machining head. In this case, the laser beam is deflected with the aid of the scanner mirror or mirrors over the surface of the workpiece, light from the surface of the workpiece being reflected back during the deflection and directed via the scanner mirror or mirrors onto an optical detector, by which the intensity of the reflected light can be measured. Since the corresponding measured light intensity values of the angular position or positions of the one or more scanner mirror(s) are sensed in an assigned form, predetermined contours on the surface of the workpiece can be detected and locally assigned with their contour edges or contour limits by the fall and/or rise of the measured intensity of the reflected light.
Since not only the shape but also the size and position of the respective machining contour are stored in the evaluation and control unit, a set-actual value comparison can be carried out with the measured position and the stored position, and the measured deviation of the shape, size or position of the contour from the set values can be compensated by intervention in the control program for the deflection of the laser beam, without a relative movement of the laser machining head and/or workpiece having to be carried out.
In the simplest and most favorable case, it may be adequate for the positional determination of the respective contour on the surface of the workpiece to carry out a deflection of the light beam along one axis, it then being possible with such a light beam deflection for at least two contour edges or contour limits to be sensed. This fact can be utilized in particular in the case of symmetrically designed contours.
Generally, however, a deflection of the light beam over the, surface of the workpiece along at least two axes will be required to determine the actual position of the laser machining head with respect to the contour formed on the surface of the workpiece or arranged there.
The axes in which the light beam is deflected may in this case be aligned parallel to or at a known angle with respect to one another, it undoubtedly being particularly favorable in many cases to carry out a deflection along axes aligned orthogonally with respect to one another in order to keep the measuring accuracy in the positional determination of the contour relatively high.
There is, however, also the possibility of deflecting the light beam by means of the scanner mirror(s) in such a way that it follows the stored predetermined contour and the deviations from the set position of the contour can be ascertained by corresponding rising or falling of the measured intensity of the reflected light with the aid of the simultaneously measured and assigned angle data of the scanner mirrors, it being possible in turn to compensate for these deviations by corresponding control of the beam deflection of the laser beam during the laser machining to be subsequently carried out.
With the aid of the values ascertained in this way, the actual position coordinates of the respective machining contour and, in the case of a known geometry and size of the respective machining contour, also its center point or centroid can then be determined, it then being possible to use the calculated center point coordinates for controlling the deflection of the laser beam for the machining.
The method according to the invention for determining the actual position of a machining contour formed on or applied to a workpiece surface can be carried out in particular in the case of symmetrical contours in such a way that, for determining the center axis, the light beam is aligned along an axis which is orthogonal to the first axis, in which the light beam is first guided. Following this, the light beam is deflected on the center axis ascertained in this way, so that the center point, for example of a circular, square or else rectangular contour, can be readily determined relatively quickly and simply. If required, this can be carried out iteratively in a number of steps.
The invention may be advantageously used for welding hollow profiles into workpieces in which openings have been formed in a way corresponding to the size and contour of the respective hollow profiles.
For this purpose, the corresponding hollow profiles can be introduced into the openings in the workpiece, normally with their end faces in line with, or forming defined edges with, the surface of the workpiece which is pointing in the direction of the laser machining head.
Once the position of the gap between the inside wall of the opening and the outside wall of the hollow profile has been ascertained with the aid of the correspondingly deflected light beam, via the evaluation of the light intensity values of the reflected light assigned to the respective pivoting angles of the scanner mirror(s), a possibly required compensation of the positional difference, between the predetermined set position and the position actually determined, can take place by correspondingly considered deflection of the laser beam. As a result, the respective hollow profile is welded to the workpiece in a positionally exact manner directly in the gap, so that a uniform, adequately solid and sealed weld can be formed.
With the solution according to the invention, hollow profiles with relatively small dimensions and thin walls can be welded with high accuracy into such a workpiece, for example a metal sheet, it being possible to use a wide variety of metals as materials for the workpiece and hollow profiles, so that there is great applicational variety.
The invention may, however, also be used for laser machining methods other than welding. In this case, the respective machining contour may be predetermined not only by a corresponding opening in the workpiece, but instead simply by the requirement that a changed reflecting property occurs at or on the corresponding machining contour in comparison with the adjacent surface of the workpiece. For example, a machining contour may be formed.as a groove-shaped depression on the surface of the workpiece.
However, a layer of a material with higher reflection or higher absorption of the light used for the light beam may also have been applied in such a way that it follows the machining contour, which has the result that, when the light beam is deflected correspondingly, there is a rise or fall in the light intensity measured by the optical detector when said light beam passes over a machining contour marked in this way.
With the method according to the invention, it is consequently also possible for correspondingly contoured openings to be cut into a workpiece or for a targeted local heat treatment to be carried out, in order for example to change .the hardness of such a workpiece in a locally targeted way or to introduce internal stress into the workpiece material in a locally targeted way.
In order to direct the light beam of the light source onto the surface of the workpiece with a relatively small light spot, it is favorable to arrange a focusing optical element, for example a convex convergent lens, in its path of rays before said light beam impinges on the scanner mirror(s).
Since the light beam should be aligned at least partially parallel to the laser beam of a laser light source, it is advantageous to arrange a beam splitter, for example an optical window, which preferably consists of ZnSe, in the path of rays of the light and laser beams. In this case, either the light beam or the laser beam is reflected at a surface of such a window and is correspondingly deflected, and the other beam, respectively, which is directed onto the window from the other side, can pass through it in a nondeflected form.
Both the laser beam and the light beam can then be directed by a focusing element, for example a focusing mirror, onto the scanner mirror or mirrors and be deflected with their aid correspondingly over the surface of the workpiece, the deflection taking place with the aid of pivoting drives of the scanner mirrors, a feedback of the respective pivoting position to an evaluation and control unit taking place with the aid of angle encoders, which are preferably integrated in the pivoting drives.
However, when there is a constant pivoting speed of the scanner mirrors, an assignment over time may also be carried out.
To simplify matters, is may be advantageous to digitize the generally analog measured values of the angle encoders with the aid of analog-digital converters and to use them in this form in the electronic evaluation and control unit for the already described set-actual value comparison, which compares the set value with the determined actual position of a corresponding machining contour.
The light of the light beam reflected at the surface of the workpiece then returns via the scanner mirror(s), through the window, as a beam splitter, and the reflected light is deflected with the aid of a further beam splitter and directed onto a fixed optical detector. The detector is in turn connected to the electronic evaluation and control unit, so that the respective measuring signal, the measured light intensity of the reflected light, can be assigned with the aid of the angle signals or the time to an exact position on the surface of the workpiece.
It goes without saying that at least the laser light source is also connected to the electronic evaluation and control unit, in order to control at least the switching on and off of the laser light source in a specifically selective manner, so that the laser machining is only carried out on the respective machining contour when the actual position of the machining contour has been determined.
It goes without saying that it is also possible to connect the light source emitting the light beam to the electronic evaluation and control unit.
The optical unit formed by the light source, optical detector and at least one beam splitter may be integrated directly in the laser machining head or be connected.to conventional laser machining heads, or be flange-mounted there, it being possible in the last case for the light beam to be directed through a corresponding.window, aligned orthogonally with respect to the light beam, into the laser machining head, onto the already mentioned beam splitter (optical window) Furthermore, the already repeatedly mentioned and described scanner mirrors with their pivoting drives and angle encoders may be integrated in the laser machining head. An additional focusing optical element may also be present there, it being possible for this to be a beam-shaping element with variable focal length.
The invention is to be described in more detail below on the.basis of exemplary embodiments.